Pendimethalin is a dinitroaniline herbicide having chemical formula [n-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine). It was first registered for use in the United States in 1974. It is a selective herbicide which controls certain broadleaf weeds and grassy weed species in crop and non-crop areas. It is applied to soil preplant, preemergence, and postemergence with ground and aerial equipment.
Pendimethalin is an orange yellow crystalline solid with a melting point of 54-58° C. It is soluble in chlorinated hydrocarbons and aromatic solvents such as methylene chloride, acetone and xylene. Pendimethalin is stable under acidic and alkaline conditions.
Pendimethalin is currently available in various different agrochemical formulation types such as emulsifiable concentrate (21.9% to 42.3%), liquid (34.4%), granular (0.7% to 2.0%), soluble concentrate/liquid (22.0%), water dispersible granules, dry flowable (up to 60.0%), capsule suspension and wettable powder (50.0%). Thus, there are many formulation choices available to a formulator setting out to prepare a desired formulation of pendimethalin.
The different formulations applicable to an agrochemical differ in their effectiveness on the desired weeds, effects the tolerance of the turf and ornamentals to the herbicide, differ in their cost advantages, differ in the drift potential of the herbicide and also differ in the ease of application and compatibility with the application equipment. Another challenge to the skilled formulator is the selection of an appropriate surfactant. Surfactants are known to increase the rate of absorption of the herbicide in the weedy species but also increase the potential for plant injury to the desirable plants during herbicide drift. Yet another challenge before a skilled formulator setting out to obtain a formulation containing pendimethalin is selecting either a granular or a sprayable formulation.
Pendimethalin has been conventionally available in both granular and sprayable forms, which may differ in the degree of weed control. However, it is often advantageous to obtain a sprayable formulation containing certain herbicides due to inherent advantages of a sprayable formulation.
The granular formulations exhibit relatively low foliar absorption because most of the applied granules fall through the leaf canopy to the soil below. In contrast, sprayable products achieve good coverage and adhere better to the foliage, providing relatively good weed control. It has also been observed that a granular product comprises a greater relative amount of the inert ingredients comparative to a sprayable formulation. Thus, the amount of the formulated product to deliver the same dosage of the active ingredient is much greater with granules resulting in higher shipping and packaging costs.
An advantage of the sprayable formulation over granules is more uniform application achieved with the sprayable formulations. The granular formulations are usually difficult to apply uniformly, especially those that contain a relatively high concentration of active ingredient. Thus, it is more often advantageous to formulate sprayable formulations of certain agrochemicals.
However, phytotoxicity of some of the sprayable formulations including pendimethalin has been reported. It is desirable to provide formulations containing pendimethalin that do not display or reduces the extent of phytotoxicity.
Another challenge that relates to dinitroaniline herbicides, pendimethalin in particular, is the potential for staining the sidewalks and other locations where the herbicide is applied. The active ingredient herbicides of the dinitroaniline class have a yellowish or yellowish-orange color. It is further known that granular formulations often do not stain badly, whereas a liquid formulation may cause more serious staining. Moreover, granules are easy to sweep or blow from concrete surfaces, whereas overspray need to be washed off before it dries. Thus, it is desirable to provide sprayable formulations containing pendimethalin that substantially reduce the incidence of staining.
U.S. Pat. No. 4,871,392, discusses under the background thereof, that pendimethalin is known to exist in polymorphic forms as orange and yellow crystals. This patent further discusses that pendimethalin is a pesticide that is difficult to formulate owing partly due to the unique staining problems associated with pendimethalin. This patent further discusses that the presence of pendimethalin in the orange macrocrystal form results in large elongated crystals in final formulations. Moreover, when pendimethalin in the orange macrocrystal form is found in compositions, very large, elongated crystals (about 3000 microns in length) appear in final product, resulting in instability, difficulty in processing and unreliability of usage apart from the severe staining further compounded by a severe clogging of nozzles.
U.S. Pat. No. 5,705,174 discloses microencapsulated pendimethalin formulations i.e. an aqueous concentrate composition of pendimethalin particles which are encapsulated by a polymeric wall material, which show a reduced tendency to form large crystals. These compositions also have improved storage stability. Unfortunately, it has been seen that microencapsulation of pendimethalin tends to slow the release of the active ingredient.
U.S. Pat. No. '174 teaches an aqueous capsule suspension formulation of pendimethalin, which contains about 456 grams of active ingredient per liter. The encapsulation of pendimethalin allows for the elimination of organic solvents in the product. The elimination of organic solvents reduces odor, staining to a certain extent, volatility and surface residue adhesion as compared to existing emulsifiable concentrate formulations of pendimethalin. Moreover, the microencapsulated pendimethalin formulation is stable under conditions of freezing and thawing and is compatible with liquid and dry fertilizer. However, the existing problem of staining, phytotoxicity and corrosiveness is not adequately addressed by the disclosure of U.S. Pat. No. 5,705,174.
Moreover, this patent essentially teaches a microencapsulated formulation of pendimethalin containing an inorganic salt, which is added prior to microcapsule formation. It is disclosed that the addition of an inorganic salt or mixtures thereof prior to the microcapsule formation provides a visibly cleaner microcapsule as more of the colored active ingredient is encapsulated, which is therefore unable to stain the external surfaces of the microcapsules. These microcapsules are also stated to be less prone to breakage than the microcapsules prepared by the conventional methods. Examples of inorganic salts taught by this patent are sodium chloride, calcium chloride, potassium chloride, sodium nitrate, magnesium sulfate and/or sodium sulfate.
However, these inorganic salts are not without additional disadvantages. The use of an inorganic salt such as sodium chloride even to the extent of 0.1-0.5% has been shown to aggravate the already existing phytotoxicity of pendimethalin. Sodium and chloride ions contribute synergistically to salt toxicity in wheat, Biologia Plantarium, 37 (2); 265-271, 1995, Martin et al., studied the effects of supplying excess mineral salts, involving sodium as a cation and a range of counter-anions, including chloride on the growth and photosynthetic capacity of a salt susceptible bread wheat. It was found that the synergistic effect of sodium and other alkali and alkaline earth metals with chlorine shows that neither of these ions is alone responsible for the salt stress induced damage in plants.
Moreover, these inorganic salts dissociate and/or dissolve in water and increase the hardness of water. The increased water hardness further reduces the temperature and suspension stability of microencapsulated pendimethalin. Moreover, the polymeric capsule wall of the disclosed microencapsulated pendimethalin is still susceptible to breakage to an appreciable extent. Thus, the problem of staining continues to plague the existing state of the art.
A need remains in the art for microencapsulated formulations of pendimethalin having improved non-staining property with reduced phytotoxicity problems. The present invention adequately addresses these and other needs existing in the art.